WordPress index.php downloading instead of being displayed

After installing WordPress on my Ubuntu 12.04 server (and seeing that it was successfully working), I tried to move WordPress into Apache’s root directory (/var/www) using the instructions found in the pre-existing subdirectory section here (http://codex.wordpress.org/Giving_WordPress_Its_Own_Directory). Unfortunately no matter what I did, trying to hit both http://tripletwollc.com and http://tripletwollc.com/index.php only resulted in my browser trying to download the index.php file instead of displaying it. I spent several hours mucking around with caches, config files, and the kitchen sink before finding out that the solution was, in my case, maddeningly simple.

I had a previous index.html file in my /var/www directory that I had renamed index.html.bak. I incorrectly assumed that this would prevent Apache from trying to use it, and simply deleting that file caused my index.php problems to vanish.